Thermal spraying, also known as flame spraying, involves the melting or at least heat softening of a heat fusible material such as a metal or ceramic, and propelling the softened material in particulate form against a surface which is to be coated. The heated particles strike the surface where they are quenched and bonded thereto to produce a coating. In a plasma type of thermal spray gun a plasma stream, formed of nitrogen or argon heated by a high intensity arc, melts and propels powder particles. Other types of thermal spray guns include a combustion spray gun in which powder is entrained and heated in a combustion flame, either at nominal velocity or in a high velocity oxy-fuel (HVOF) gun. In a wire type of gun a wire is fed through a combustion flame where a melted wire tip is atomized by compressed air into a fine spray for deposit. A two-wire arc gun melts contacting wire tips with an electrical arc for atomization by compressed air.
Various types of traversing equipment have been taught or used to traverse or scan a spray deposit over a relatively large substrate to produce as uniform a coating as practical. These include equipment designed to traverse and index the gun automatically in an x-y plane over preset areas, and robots with multiple linear and rotational axes particularly for complex shapes.
Uniform thickness is easier to achieve for coating a shaft which may be rotated at high speed while the spray gun is traversed back and forth along the shaft. On flat or large curved areas the gun generally is waved or moved back and forth while it is traversed in a direction generally perpendicular to the waving motion. A problem is that, during a cycle, the gun must be stopped at each end of the wave to reverse direction. The spray stream lingers longer near each of these points causing a much thicker layer to be deposited at each end. An additional problem is a hot spot that can develop at each point of lingering, thus overheating the coating and substrate to cause detrimental oxidation and other metallurgical changes. This is particularly acute for an HVOF type of gun which produces a relatively narrow spray stream and small deposit spot.
The gun can be moved off the edge of the substrate for each reversal, but this results in loss of spray material which can be expensive, and can require masking to prevent unwanted areas to be coated. Multiple cycles of traversing with overlapping layers have been utilized for smoothing out the thickness variations, but often with only partial success because of the complex programming required to compensate a varying thickness profile. Such programming is even more extensive for complex shapes. Even programming of a robot can be time and memory consuming, and thus quite expensive for each different type of substrate to be coated.
Other patterns for the motion have been utilized, such as circular, oval or figure eight to reduce the problems of the lingering and non-uniform thickness. Circular and oval patterns result in substantially thicker coatings at the edges when the patterns are traversed. Figure eight patterns are better in this regard.
A figure eight pattern with uniform velocity of travel of the deposit may be programmed into a robot, but this was found by the present inventors to be extremely complex and time consuming and, therefore, is believed not to have general practicality.
Mechanisms such as with linked gearing and arms or cams can produce a figure eight motion which is an improvement over linear waving motion with regard to deposit thickness. However, simple mechanisms typically have variations in velocity of travel along the configuration of the figure eight, particularly slowing down along the distal ends of the figure, thus negating some of the advantage. Thus such mechanisms do not fully solve the problem.
A thermal spray gun, particularly an HVOF type, should have its deposit spot moved along a substrate at a relatively high velocity. Any apparatus dedicated to moving the gun must be quite robust, and should remain simple to achieve this.
An object of the invention is to provide a novel apparatus for moving a thermal spray gun over a substrate, particularly a large area substrate. Another object is to provide such an apparatus for producing improvements in uniformity of coating thickness. Yet another object is to reduce overheating of spots in the coating during deposition. A further object is to provide such an apparatus for moving a thermal spray gun in a figure eight motion, with improvement wherein the travel of the deposit along the figure eight has reduced non-uniformity in velocity, with corresponding reductions in non-uniformity in coating thickness and in heating during deposition. Another object is to provide such an apparatus that is robust and capable of continuous, rapid motions.